Web Services Proxy is an extendable intermediate that can monitor and control XML communication with good performance. It adopts a plug-in architecture enhanced by characteristic techniques named "MFI4P" and "XPath centric architecture." MFI4P efficiently controls plug-in libraries over various XML operations by optimizing transformation cost of XML between the libraries. The XPath centric architecture performs XML processing better and makes it easy to develop plug-in by using Streaming XPath and "two phase rule evaluation." Experimental results show many types of management functions can be built as plug-ins and XML processing overhead is minimized in many cases.